Who gets cold when diving in cold water, and why?
Is cold something you can get used to? How?

BRIEF OVERVIEW OF SUSCEPTIBILITY TO COLD
Just as the "dose" of nitrogen or oxygen, meaning partial pressure
and time exposed, are main factors in decompression sickness and oxygen
toxicity respectively, major factors in cold stress are temperature and
length of exposure. As with dosage of any drug or substance, several
interacting
and competing mechanisms, in addition to the dosage, determine if you
will
be affected, and by how much.

You generate heat in many ways, lose heat in many ways, and have
various
anatomical structures and physiologic schemes to block heat loss and
gain.
Thermal scientists can put all the figures for heat generation, heat
loss,
and resistance to heat loss into mathematical models to estimate what
your
final temperature might be. But it is more involved than just saying
that
young or old, or big or small people have any one characteristic and
therefore
greater susceptibility.

Losing body heat, by itself, does not mean that you are chilling.
You
lose heat all the time. Your body generates heat in the process of
being
alive. If you didn't lose heat, your body would cook (sometimes it
does,
and that will be covered in another article on overheating).

Losing heat doesn't necessarily mean you are in danger of
hypothermia
or any other injury from cold. You need to lose some heat. Whether you
stay comfortable or get cold depends on how much heat you keep and how
much you lose.

You lose heat in several ways. You also generate heat and store
heat.
No one variable such as gender or skin surface area makes anyone more
susceptible
to chilling or hypothermia than anyone else.

Losing body heat, by itself, does not mean that you are chilling.

FACTORS IN SUSCEPTIBILITY

SURFACE AREA TO MASS RATIO. Much is made of the "surface
area
to mass ratio." What is it? Your heat production roughly proportional
to
your body mass. On the other hand, radiation of heat from your body to
the environment is in proportion to the area of your skin that covers
you.
The proportion of how much external surface area you have compared to
how
much internal mass is your ratio.

Car and home heat-redistributors are built to have long thin
shapes.
Their high surface area-to-mass ratio gives off, or radiates, lots of
heat.
Imaginatively, they are called radiators. Long, thin spaghetti cools
rapidly.
Short, round, bulky, baked potatoes stay hot longer. Like spaghetti,
your
fingers and ears are long and thin with much exposed surface. Fingers
and
ears chill faster than your torso. Fingers have less total surface than
your torso, but a higher ratio. Your torso, very much like a potato,
has
more internal mass compared to its outer surface of skin, giving it a
lower
ratio of surface area-to-mass.

Bodies, and body parts, that have a large surface area compared to
their
mass, can radiate more heat than those with smaller surface
area-to-mass
ratios. Although a higher ratio does allow more relative heat loss, it
is not the main determinant of chilling. Someone with a larger ratio
can
lose more heat through that particular pathway yet still not be at
greater
risk of chilling, because of all their other heat-conserving and
generating
mechanisms. Moreover, a larger person has more total surface area and
loses
more total heat than a smaller person. For example, a large male has
more
total surface area, and so loses more total heat than a smaller man or
woman, but is not more susceptible to chilling for that one reason.

AGE. Young children are less able to thermoregulate in the
cold
than adults for a variety of reasons including size, active heat
generation,
vasomotor control, and other factors. Risk of chilling also generally
increases
with aging, although changes in physical fitness and body composition
that
accompany aging, is often confused for aging itself.

BEHAVIOR. Is the person who gets out of the water first,
really
the cold one? A thermal stress workshop held at the Institute for Naval
Medicine in England by the Diving Medical Advisory Committee discussed
what they called the non-responder to cold. They stated, "It is still
not
known what the differences are between the man who responds to and
complains
of the cold, and another man who cools and is unaware that he is
cooling.
Presumably this latter type of diver is a potential hypothermic
casualty."

MEDICATIONS. Medications called beta blockers are commonly
prescribed
for migraine headache. They are also sometimes taken for high blood
pressure,
although other medications have gained greater acceptance as
anti-hypertensives.
People taking beta blockers sometimes report reduced cold tolerance. A
possible reason is that beta blockers, particularly a class called
non-selective
beta blockers, were found in some studies to block non-shivering
thermogenesis,
which is one small means of heat production.

EXERCISE. Contrary to popular belief, you won't always get
colder
by exercising in cold water. Both heat loss and heat production
increase
when exercising in cold water. Whether you get cold or warm depends
which
you have more of. Often the exercise can generate enough heat to
overheat
you, as USNavy divers found out during Desert Storm operations in the
Gulf.

FITNESS. Your thermal tolerance can improve with physical
fitness,
although cold tolerance better increases with exercise in cold
conditions
than from exercise alone. In other words, to get used to the cold, you
need to be out in it. Often.

PROTECTIVE CLOTHING. Clothing studies yield interesting
results.
Subjects' core temperatures are sometimes lower with protective
garments
than without. Lack of input from cold receptors in their hands,
decreased
the body's ability to make the needed blood flow changes necessary for
cold protection. Sensory information from cold receptors in the
extremities
seems of high importance in thermoregulation. Still, protective
clothing
is important, and makes a life-and-death difference in extreme cold air
and water. Protective clothing protects you from losing more heat than
you can replace.

GENDER. Women are not more susceptible to hypothermia than
men,
as commonly thought. To the contrary, several studies show women are
often
less susceptible. On average, women have better ability to limit heat
loss.
They may generate less (and sometimes more) total heat than men
depending
on work load, fitness, body size, and other variables. Men, on average,
usually lose more total heat from higher skin temperatures due to their
lesser vasoconstrictor response (evidenced by often warmer hands), and
from their larger total skin surface area, and for that reason, must
counter
with increased heat production from typically greater mass and
metabolism.
It takes more calories and metabolic work to keep up such heat
production,
making a very extreme survival situation more problematic for males -
they
may be more likely to starve and freeze. Evidence is strong that women
protect their core temperature in the cold as well or more than men.

What about the warmer hands issue? That doesn't mean that men fare
better
in the cold. It indicates that women are losing less heat through their
periphery. Men's warm hands pour heat out into the environment. Your
skin
temperature is not 98.6F (37C). That familiar number is the average
temperature
of your core. Skin temperature is far cooler than core temperature. One
of the ways your body resists heat loss through your periphery is by
reducing
warm blood flowing to your skin surface. In the cold, your skin
temperature
quickly drops to that of (or close to) the surrounding air or water. If
skin surface temperature is close to surrounding temperature, the
gradient
is small, so heat loss is small. (Heat travels down gradients from high
to low, just as with nitrogen load.) People with cooler skin in the
cold
have a smaller skin-to-environment gradient to lose heat. An analogy is
if you stand outside your house in cold weather, touch the exterior
wall
and find it warm, you would notice the expensive loss of heat and know
your home needed better insulation. You may even wonder who designed
such
an inefficient structure.

BODY SIZE AND SHAPE. A large person can produce and
store
more heat than a smaller person. Adaptations in body shape and size,
hypothesized
to aid survival as a species in cold climates, is summarized in
Bergman's
rule. Bergman's rule is a generalization that peoples originally native
to cold climates are larger than those from warmer climates.

Now imagine a long, tall, slender person. With large body size,
arm
and leg length often increase. More heat is lost through these areas of
high surface-to-mass ratio, and comparatively little fat insulation.
Another
generalization, called Allen's rule, takes limb length into account.
The
short arms and legs of large people from cooler regions, for example
Eskimos,
helps reduce heat loss.

Body size and shape contribute to susceptibility to cold, but,
like
any other individual factor, do not determine it.

FACTORS IN SUSCEPTIBILITY TO COLD

Water and air temperature
Duration
of exposure
Skin temperature
Body composition
Very young and very old age
Certain medications
Protective garments
Physical work load
Body size
State of acclimatization
Fatigue
Hydration
Nutritional status

WHAT IS ACCLIMATIZATION?
Cold acclimatization is a well-documented process of gradually
increasing
your resistance to cold injury through regular cold exposure. Following
the recommendation of the International Union of Physiological
Sciences,
the term acclimatization is distinguished from acclimation.
Acclimatization
means change from seasonal or geographical exposure; acclimation is
change
produced in a laboratory.

WHO ACCLIMATIZES TO COLD?
Major examples of geographic acclimatization to cold are the indigenous
people of the African Kalahari, the Australian desert, and Tierra del
Fuego
in Southern Chile. Many sleep outdoors nearly naked in freezing
temperatures.
Seasonal acclimatization occurs in people working outdoors year round
and
fishermen who dunk their hands in cold water all winter to tend their
nets.
Divers continuing to work late into the winter season, or year round in
cold waters, gradually increase their cold tolerance. Extent varies
among
individuals and with exposure.

WHAT CHANGES OCCUR IN TRUE ACCLIMATIZATION?
True cold acclimatization involves at least three adaptations.
Cold-acclimatized
people begin shivering at lower body temperatures, because they
generate
more heat without shivering. A big hallmark of cold-acclimatized people
is improved ability to sleep in the cold. Cold acclimatization may
involve
either increased or decreased skin temperatures, depending on
circumstances.
In some cases, skin blood flow increases to keep extremities warm and
to
resist cold injury. In other cases it decreases to reduce heat loss.
For
example, skin temperatures of Australian Aborigines were lower while
sleeping
than those of the unacclimatized European investigators.

LOSING YOUR EDGE
When chronic exposure to cold environments ends, you gradually lose
your cold adaptation. When acclimatized Korean divers switched from
bathing
suits to wet suits, their thermal advantage decreased. Loss of
acclimatization
was also documented in the Ama divers of Japan when they began wearing
cotton suit insulation and wet suits.

ACCLIMATIZATION IS NOT ALL THERE IS TO DIVING WARM IN THE COLD
To truly acclimate to cold weather, you need to expose yourself to
cold conditions on a regular basis, and to exercise in the cold. You
will
reduce or eliminate your acclimatization potential if you keep yourself
in a tropical micro-climate of warm clothing and indoor heating.

How practical is it to live a cold life in order to acclimatize to
cold?
Up to a point, it helps greatly. Below critical environmental
temperatures,
obviously, acclimatization is not all there is to diving warm in the
cold.
Cold affects many of your body systems as they make adjustments to
increase
heat production and decrease heat loss. Extreme cold exposures
overwhelm
your protective systems, with chilly effects.

One important way to conserve heat and tolerate cold water
immersions
is to wear good thermal protection. Various animals dive in Arctic
waters
using both wet suit and dry suit technology. The fur of seals and polar
bears, for example, is an effective wet suit. It adds exterior
insulation
to their thick fat layer by trapping a two to ten millimeter water
layer
near their skin. The feather pelt of penguins, on the other hand, works
like a dry suit, maintaining an insulating layer of air. Humans who
have
no feathers or fur should wear exposure suits that include head
covering
when they dive in cold water.

Some divers ask if pouring warm water in your wet suit, or warming
up
between dives in a heated car or boat cabin, will cause you to sweat
and
vasodilate your peripheral blood vessels, increasing heat loss, thereby
making you colder than before. It's unlikely that you will overheat to
such an extent. The additional heat you gain back is important for
rewarming.
You will be warmer than before and will build back a heat reserve.
Rewarming
is an important part of cold water diving.

YOU CAN DO SEVERAL THINGS TO CONSERVE HEAT WHILE DIVING IN COOL
AND
COLD WATER:

Wear
good exposure garments, suitable for conditions
Get
the weather report and make site condition checks
Allow
wider diving safety margins with colder conditions
Stay
well nourished, rested, and hydrated
Pre-wet
your face and hands
Get
in slowly
After
diving, dry off, get changed, and get out of the cold
Rewarm
well between dives
Keep
in good muscular and aerobic shape to improve your
heat-conserving
and heat-producing systems
If
you are cold, do something about it

DON'T JUST SIT THERE
Diving safely in the cold is a matter of not losing more heat than
you produce. Divers rarely get clinical hypothermia from diving, but
often
get cold and uncomfortable, which can affect fun and safety.

If you are cold, do something about it. Safety in the cold
requires
action and thought by the diver before, during, and after diving. You
can
dive safely in cold water when you properly prepare.

For more information on cold and other diving physiology topics,
see
the fun book, DIVING PHYSIOLOGY IN PLAIN ENGLISH. This book
covers
decompression theory, tables, computers, effects of immersion, DCS, O2
toxicity, lung injuries, heat and cold, swimmer's ear, marine stings,
exercise
and nutrition, headaches, why you have to 'pee' when you get in the
water,
and many other topics. Fifth revised printing 2003.